Here I present util how to handle with Pandemic mechanic, since I was unable to find any solution.
With a help of #WoWUIDev, this is the Pandemic util I made for my indev Warlock addon (note that the following util uses global Tooltip, but it's just for clarity):

Finally, I've added a handy Tooltip.GetUnitAuraPandemic that takes same arguments like UnitAura, and returns if Spell duration already benefits from Pandemic or not.
Example: Before casting Siphon Life on target:

Even setting aside the conceptual flaws (your code is way more complicated than it needs to be to do what it does) there are several basic issues with your code that make it pretty unsuitable as an example of how anything should be done.

Lua Code:

Tooltip = Tooltip or{};

This creates a global variable named "Tooltip". You say you know this, yet you did it anyway. This is 100% bad and you should never do it for any reason. Doing it in something you're calling an example for others to use is even worse than doing it in code for yourself.

This creates a global variable named "Tooltip._tooltip" which, though slightly less bad than a global "Tooltip", is still not good. Global variables should always follow two simple rules:

1. It should be unique, so there is no chance of it colliding with another badly named or accidentally leaked global. At best, global variable collision will make an addon break. At worst, it will break the entire UI and make the game unplayable.

2. It should clearly identify the origin (ie. what addon created it) and purpose of the value it contains, so if it shows up in an error message, stack trace, /framestack tooltip, or other debugging context, there is some hope of the user figuring out where it's coming from.

Lua Code:

Tooltip.GetTooltip = Tooltip.GetTooltip orfunction(itemLink)

Since this code is running in the main chunk of your addon, it's only ever executed once. There will never already be a "GetTooltip" method on your "Tooltip" object, so there's no point in checking for one before defining it.

Same as above. No reason to check for things that you already know don't exist.

Lua Code:

ifnot text thenbreakend;

While this won't break anything in the context of scanning an aura tooltip, it's also not necessary in that context -- if tooltip:NumLines() is 3, then the tooltip has 3 lines with text; you don't need to verify that there's text -- and will break scanning in other types of tooltips, since item tooltips, for example, can have empty lines in the middle of them.

Lua Code:

returnnil;

Writing this at the end of a function does absolutely nothing, and there's no reason to write it. It just takes up space for no reason.

Even setting aside the conceptual flaws (your code is way more complicated than it needs to be to do what it does) there are several basic issues with your code that make it pretty unsuitable as an example of how anything should be done.

Lua Code:

Tooltip = Tooltip or{};

This creates a global variable named "Tooltip". You say you know this, yet you did it anyway. This is 100% bad and you should never do it for any reason. Doing it in something you're calling an example for others to use is even worse than doing it in code for yourself.

This creates a global variable named "Tooltip._tooltip" which, though slightly less bad than a global "Tooltip", is still not good. Global variables should always follow two simple rules:

1. It should be unique, so there is no chance of it colliding with another badly named or accidentally leaked global. At best, global variable collision will make an addon break. At worst, it will break the entire UI and make the game unplayable.

2. It should clearly identify the origin (ie. what addon created it) and purpose of the value it contains, so if it shows up in an error message, stack trace, /framestack tooltip, or other debugging context, there is some hope of the user figuring out where it's coming from.

I though that my note about tainted global is clear enough.
It's just for clarity:

Originally Posted by Benio

note that the following util uses global Tooltip, but it's just for clarity

If you wish implementing this util in your addon, then sure, you should follow all those tainting rules.

Originally Posted by Phanx

Lua Code:

Tooltip.GetTooltip = Tooltip.GetTooltip orfunction(itemLink)

Since this code is running in the main chunk of your addon, it's only ever executed once. There will never already be a "GetTooltip" method on your "Tooltip" object, so there's no point in checking for one before defining it.

Same as above. No reason to check for things that you already know don't exist.

It's util. Util (or utility) is indented to be used multiply times.
This means, that it is highly propable that a few addons will use sam utility.
Then we can just use already existing one.

Originally Posted by Phanx

Lua Code:

ifnot text thenbreakend;

While this won't break anything in the context of scanning an aura tooltip, it's also not necessary in that context -- if tooltip:NumLines() is 3, then the tooltip has 3 lines with text; you don't need to verify that there's text -- and will break scanning in other types of tooltips, since item tooltips, for example, can have empty lines in the middle of them.

Optimalization. This util is indented to be used to retrieve spell info. No spell I am working with in my indev addon have empty line before mentioned duration. They have empty line after it and thus, it's worth skipping this.

Originally Posted by Phanx

Lua Code:

returnnil;

Writing this at the end of a function does absolutely nothing, and there's no reason to write it. It just takes up space for no reason.

Clarity. Some people likes to see what function returns. However you're absolutelly free to remove it, if you only wish.

I though that my note about tainted global is clear enough.
It's just for clarity:

Not good enough. If you're going to call it an example, it should be a good example of working code. People will copy/paste "example code" without understanding it fully (or at all) and if your code does things wrong, so will theirs. Doing things wrong doesn't provide "clarity" -- it's just "doing things wrong".

Originally Posted by Benio

It's util. Util (or utility) is indented to be used multiply times.
This means, that it is highly propable that a few addons will use sam utility.
Then we can just use already existing one.

If you intend for this to be a reusable library, write it as a proper library and make an actual release instead of a forum post.

If you intend for the code to be copied and pasted into addon files, it should be entirely local, and not create any globals at all. There's not nearly enough overhead to justify putting this in the global namespace, especially with some of the worst global names possible.

Originally Posted by Benio

Clarity. Some people likes to see what function returns. However you're absolutelly free to remove it, if you only wish.

Again, this doesn't provide "clarity". It just adds clutter and confusion. When you post "example code" as a snippet in a forum post, you should be aware that 99.9999999999% of the people who are going to use it will not have a good understanding of the language or the API, and will just use the code as-is, probably without even reading it. Using bad coding paradigms and redundant/unnecessary/inefficient code is the opposite of helpful.